Millimeter wave imaging system

ABSTRACT

A millimeter wave imaging system that includes at least one millimeter wave frequency scanning antenna for collecting frequency dependent beams of millimeter wave radiation from a narrow one-dimensional field of view. The collected radiation is amplified at the collected frequencies and the amplified signals are separated into frequency dependent bins with a tapped-delay beam-former. These bins are then sampled to produce a one-dimensional image of the antenna field of view. A two dimensional image of a target may be obtained by moving the target across the field of view of the scanning antenna. In a preferred embodiment the antenna is only 4.5 inches in length and constructed from WR-10 waveguide with inclined slots cut in one of the narrow walls at 79 mil spacings. This geometry creates a frequency-scanned antenna spanning a 20 degree vertical field of view over a 75.5-93.5 GHz operational band of the sensor, starting at approximately 1 degree below horizontal at 93.5 GHz and ranging to approximately 21 degrees below horizontal at 75.5 GHz. In this embodiment 64 of these antenna elements are arranged in four stacks of 16 antennas focused at about 18 inches to construct a portal contraband screener.

The present invention is a continuation in part of U.S. patentapplication Ser. No. 09/965,875, filed Sep. 28, 2001 which isincorporated by reference herein. The present invention relates toimaging systems and in particular to millimeter wave imaging systems.

BACKGROUND OF THE INVENTION

Imaging systems operating at millimeter wavelengths (1 cm to 1 mm; 30GHz to 300 GHz) are well known. These systems can be important becauselight at these wavelengths is not completely attenuated by substantialdistances of fog or smoke, as is visible light. Light at millimeterwavelengths will also penetrate clothing and significant thickness ofmaterials such as dry wood and wallboard. These millimeter wave imagingsystems have therefore been proposed for aircraft to improve visibilitythrough fog and for security applications for detection of hiddenweapons and the like. Such systems are described in U.S. Pat. Nos.5,121,124 and 5,365,237 that are assigned to Applicant's employer. Thesystems described in those patents utilize antennas in which thedirection of collected millimeter wave radiation is a function offrequency. This type of antenna is referred to as a “frequency scanned”antenna. The collected millimeter wave light is analyzed in a spectrumanalyzer to produce a one-dimensional image. In the systems described inthe '124 patent the antenna signal is used to modulate an acousto-opticdevice (a Bragg cell) that in turn modulates a laser beam to produce aspectral image. In the systems described in the '237 patent anelectro-optic module is modulated by the antenna signal and theelectro-optic module in turn modulates the laser beam to impose themillimeter wave spectral information on a laser beam that then isseparated into spectral components by an etalon to produce an image.

U.S. Pat. No. 4,654,666 describes an imaging system which includes afrequency scanning antenna and a spectrum analyzer for converting codedradiation distributions collected by the antenna into a time codeddistribution so that a one-dimensional scene can be reproduced.

The systems referred to above are complicated and costly to construct.What is needed is a relatively low cost, easy to operate millimeter waveimaging system, especially for use in portal screening for contraband.

SUMMARY OF THE INVENTION

The present invention provides a millimeter wave imaging system thatincludes at least one millimeter wave frequency scanning antenna forcollecting frequency dependent beams of millimeter wave radiation from anarrow one-dimensional field of view. The collected radiation isamplified at the collected frequencies and the amplified signals areseparated into frequency dependent bins with a tapped-delay beam-former.These bins are then sampled to produce a one-dimensional image of theantenna field of view. A two dimensional image of a target may beobtained by moving the target across the field of view of the scanningantenna.

In a preferred embodiment the antenna is only 4.5 inches in length andconstructed from WR-10 waveguide with inclined slots cut in one of thenarrow walls at 79 mil spacings. This geometry creates afrequency-scanned antenna spanning a 20 degree vertical field of viewover a 75.5-93.5 GHz operational band of the sensor, starting atapproximately 1 degree below horizontal at 93.5 GHz and ranging toapproximately 21 degrees below horizontal at 75.5 GHz. In thisembodiment 64 of these antenna elements are arranged in four stacks of16 antennas to construct a portal contraband screener. A narrow,rod-shaped cylindrical lens covers the waveguide slots at each elementand vertically focuses the antenna beam 18 inches from the antenna. Theantenna segments are aligned along one focal axis of a verticallyoriented elliptical cylinder reflector, 4.5 inches across with thesecond, parallel focal axis of the reflector located 18 inches from theantenna. This arrangement gives a 2-dimensional beam focus at 18 inches,with an approximate depth of focus covering 14 inches at minimum rangeto about 29 inches at maximum range. The frequency-scan angular rangecorresponds to about 4.5 vertical inches at the minimum operationalrange of 14 inches. The horizontal and vertical resolution (half-powerbeam-width) at the center-band frequency of 84.5 GHz is about 1.57degrees, or less than ½-inch at the 18-inch focus. Each of the fourstacks of antennas is directed at regions of a portal to permitcomposite front, side and back millimeter wave imaging of personspassing through the portal. The fixed antenna elements provide thevertical scan and the passage of the person provides the horizontalscan.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1C show features of a one-dimensional frequency scanning slotantenna.

FIGS. 2A and 2B show techniques for focusing the antenna shown in FIGS.1A-C.

FIGS. 3A and 3B show an arrangement 64 of the above antennas in a portalcontraband scanner.

FIG. 4 shows electronic circuits for converting millimeter wave signalsto images.

FIG. 5 shows a person passing through a screening portal.

FIG. 6 shows an embodiment with a large depth of field.

FIGS. 7A and 7B show the relationship between resolution and distancebetween antenna and target.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Antenna Element

FIGS. 1A, 1B and 1C are drawings showing features of a one-dimensionalmillimeter wave antenna according to aspects of the present invention.FIG. 1A shows the elevation field of view of the basic antenna element2. Each antenna element 2 is constructed of WR-10 waveguide has exteriordimensions a=0.180″, b=0.130″ and interior dimensions a=0.100″,b=0.050″. In fabrication of the antenna, one of the narrow walls isthinned from 40 mils to 6 mils. Then, each WR-10 waveguide antenna has57 inclined slots 4 cut into its narrow wall at a spacing of 0.079″,which serve as emitting elements. The angle of the slots, and thus thecoupling coefficient, increases from 9.66 degrees on the feed end toover 25 degrees at the load end to provide nearly constant fieldstrength along the antenna length. The direction of the anglealternates, providing a “pi” phase shift between successive couplingslots. This geometry creates for a vertical mounted antenna a frequencyscan spanning a 20-degree vertical field of view over a 75.5 to 93.5 GHzoperational band of the sensor starting at minus 1 degree with thehorizontal at 93.5 GHz and ranging to approximately 21 degrees below thehorizontal below the horizontal.

Focusing the Antenna Element

In preferred embodiments the antenna element 2 shown in FIGS. 1A, 1B and1C is focused to 18 inches as shown in FIGS. 2A and 2B. In this case avertically oriented elliptical cylindrical mirror 8, 4.5 inches acrosshas one of its focal lines at the center of slots 4 of antenna element 2and its second focus at 18 inches from the antenna as shown in FIG. 2A.This focuses the antenna beam 8 in the horizontal direction at 18inches. Also a narrow rod-shaped collimating lens 6 covers waveguideslots 4 and vertically focuses the antenna beam 8 at 18 inches from theantenna. At the frequency range of 93.5 to 73.5 GHz the wavelengths ofthe collected radiation are about 0.14 inch (3.6 mm) corresponding tothe mid-frequency, 83.5 GHz. However, in this preferred embodiment theresolution element (as described in more detail below) is somewhatlarger (in the range of about 0.5 inch) in both the horizontal andvertical directions. Antenna element 2 along with its focusing elementsare designated as antenna element 50 in FIGS. 1A, 2B, 3A and 3B.

Antenna Electronics

Calibration and Amplification

In this preferred embodiment Dicke switching is used for calibration ofthe imaging system. This technique utilizes, as shown in FIG. 4 afront-end switch 20 that alternates between looking at the antennasignal and looking at a set temperature load 21. The front-end switch 20switches at a rate of 3.84 kHz between the antenna and a loadtermination. The load can be heated by approximately 40K for oneswitching cycle. This allows the unit to perform a two-temperaturecalibration in real time, compensating for gain fluctuations in theamplifiers as well as temperature offsets. The switch itself is a W-bandmicrowave monolithic integrated circuit (MMIC) PIN switch fabricated byM/A-Com with a transmission loss of 1.8 dB.

The front-end switch is packaged with, and feeds directly into, afront-end amplifier unit 22. This front-end amplifier unit consists oftwo low noise MMIC amplifiers 22A and 22B, band pass filter 22C and lownoise MMIC output amplifier 22D. The amplifiers are required to have awide bandwidth but maintain a low noise figure, as they set the noisetemperature for the entire system. They operate over an 18 GHzbandwidth, from 75.5 GHz to 93.5 GHz. The first two amplifiers in thechain 22 A and 22B have a noise figure of approximately 4 dB over the 18GHz band and a gain of about 19 dB. Band pass filter 22C separates theseamplifiers from the third amplifier 22D that is of a slightly differentdesign. The third amplifier 22D, the output amplifier, is tuned for again of about 22 dB and is capable of output powers of up to 1 mW beforecompressing. Once packaged, the entire gain stage has approximately 53dB of gain and a noise output power of −11 dBm, with a noise figureaveraging 7 dB. This includes losses from the filter and thetransitions. Thus, each amplifier channel 22 provides 53 dB of gain, aswell as an integrated matched load with a heater, and PIN switch forin-situ two-temperature flat field calibration. The MMIC amplifiers andthe band pass filter are preferably fabricated using a co-planerwaveguide design on an indium phosphide substrate.

Tapped Delay Beamformer

This broadband amplified antenna power is fed into a tapped-delaybeamformer as shown in FIG. 4 for decomposition into frequency binsrepresenting a vertical frequency image of the antenna field of view.Delay lines 26 transmit the amplified antenna signal into 32 signalports of beamformer 24. Beginning at port 24-1 at the left side ofbeamformer 24, the signal to each port is delayed by 36 ps (relative toits left side neighbor port). The 36 ps delay is equivalent to threewavelengths at centerband of 83.5 GHz. (The millimeter wave frequency of83 GHz corresponds to a wavelength of about 3.6 mm and light travelsthat far in about 12 ps.) Thus, a signal arriving at port 24-1 at time 0would also arrive at port 24-2 at time 36 ps, would arrive at port 24-16at time 576 ps and would arrive at port 24-32 at 1.152 ns. The series of32 taps samples a total time interval of 1.152 nanoseconds, yielding afrequency resolution of 870 MHz for the beamformer. (The frequencyresolution for these beamformers is the inverse of the total timespread; so in this case 1/1.152 ns=870 MHz.) The beamformer sorts the18,000 MHz broadband signal from the antenna into 32 frequency bins atan average separation of 580 MHz, thus over sampling the vertical focalplane by about 2.4X relative to the 1400 MHz bandwidth of each antennabeam. (The frequency separation is 18,000 MHz/31=580 MHz, and thebeamwidth of the antenna beams is approximately equal to the inverse ofthe time [about 0.71 ns] for light to traverse the antenna element.)Losses in the delay line, the beamforming lens, and input transition, aswell as bandwidth splitting losses drop the power level at each lensoutput to about −36 dBm. A set of 32 sensitive detector diodes 30integrates this power in the 32 frequency bins for each channel toprovide a voltage signal corresponding to the intensity of themillimeter wave light collected by the antenna element at each of the 32frequency ranges. The voltage signal from each of these diode signals isthen read out by multiplexing readout integrated circuit chips onreadout integrated circuit board 32.

The beamformer is implemented in a low loss dielectric, such aspolypropylene, with smooth copper cladding. The delay lines 26 arecreated at very low cost with a lithographic etching that creates thecircuit pattern, which is then sandwiched between two ground planes in aheated press. In preferred embodiments, the smoothness of the coppermaking up the inside surface of the delay lines is extremely important.Applicants have discovered that they could reduce the losses in theselines from 1.2 dB/inch to about 0.5 dB per inch by requiring that thecopper surface roughness not exceed 300 nanometers. Prior art microwavesurface roughness specifications were 1400 to 2900 nanometers. Asexplained above, the signals from these 32 taps are launched intobeamforming lens 24, which steers the beam into one of 32 output ports28 based upon the signal frequency.

Portal Contraband Screener

In a preferred embodiment of the present invention shown in FIGS. 3A and3B 64 antenna elements of the type discussed above are utilized toprovide a portal contraband screener. In this preferred embodiment foursets of 16 vertically stacked antenna elements 50 are arranged tomonitor persons passing through a portal preferably on a horizontalescalator at a known velocity of about 1.5 feet per second. The stacksare 80 inches high with two of the stacks 10A and 10B arranged to viewthe person's front and side and two of the stacks 10 C and 10 D arrangedto view the person's side and rear all as shown in FIGS. 3A and 3B.

As a person 51 approaches the portal, which is about 41 inches wide;he/she enters the area of focus at a distance equal to roughly half theportal width (20.5 inches) from the portal centerline. At this range theforward-looking imaging antennas in stacks 10A and 10B are focused onthe subject's front midline. As the person moves closer to the portal,the sensor foci sweep outward from the midline as indicated at 52 togenerate a full 2-D image of the front and sides of the person. At thenominal travel speed of 1.5 feet per second, the antenna beam movesthrough one resolution element every 39.2 milliseconds. The imager readsout at 30 Hz, slightly over sampling the horizontal plane. In onesecond, as the person moves forward by 18 inches, the two antennascombine to record 60 columns of image pixels surrounding the front andtwo sides of the subject, separated horizontally by less than aquarter-inch projection. As the person leaves the portal, a second pairof antennas in stacks 10 C and 10 D images his/her back and sides in thesame manner.

Electronic Features of the Contraband Screener

In this embodiment, the 16 antenna elements making up each antennacolumn feed 16 receiver channels, which are configured into two8-channel packages (“octapaks”). These octopaks include an amplifier setand beamformer for each antenna. The amplified signals from the antennaelements are processed as a pair of images, one representing the frontand sides of the person and the other representing the sides and rear ofthe person passing through the portal. In this preferred embodiment thesensors operate at a 30 Hz rate, producing 30 images per second. If weset the passage so that the image time for both front and rear imagestake one second each, both front and rear images will each contain 60pixels in the horizontal direction. For the vertical direction, each ofthe 16 antenna elements in each column produces 32 angular beams for atotal of 512 angular beams. These beams will be equally spaced in thevertical direction over 80 inches only at about 14 inches from theantenna stacks and will overlap beyond about 14 inches. Thus, both thefront and rear images will each contain about 60 pixels across and 512pixels high, and the images will produce a wraparound view of the persontraversing the portal. The pixel size is about 0.5 inch in thehorizontal direction and about 0.16 inch in the vertical direction at arange of 14 inches from the stacks. For those portions of the personlocated substantially farther from the antenna stacks than 14 inches,the pixel data should preferably be modified with computer software toaccommodate the overlap to produce the wrap-around images.

Each of the two octopaks in each column holds eight switches and gainstages with WR-9 inputs. Each octopak includes connections for power andcontrol signals and plenty of shielding to prevent feedback in the gainstage. Each octopak measures 1.5″ long, 0.825″ wide, and weighs 2.25 oz.

Background and Illumination

When a person is not passing through the portal, the antenna arrays havenothing within their focal area and instead receive signal from a broadarea beyond the focal region. This area can be coated with millimeterabsorptive foam at ambient temperature. The foam acts as a blackbody atmillimeter wave frequencies, emitting a fixed, broadband signal to theantennas. If the foam temperature is less than the temperature of ahuman body, the foam provides a good contrast to a person passingthrough the detector. This improves the clarity and sharpness of thegenerated images. Also, in preferred embodiments contour contrast can beadded to the images of the persons being screened by providing a coldsurface above the portal that would be a source of low temperaturethermal radiation. Therefore, millimeter radiation in the band detectedby the antenna elements that is reflected off the person after beingemitted from the cold source will be very small compared to reflectedradiation from other surrounding warmer sources. As a result the scannerwill see substantial contrasts on the persons scanned depending on theangular orientation of various portions of his body, his clothing andpotential contraband.

Other Embodiments

Persons skilled in the art of contraband detection will recognize thatmany modifications can be made to the examples presented above. Forexample, instead of having the person pass through the portal on ahorizontal escalator as described above, the person could be required towalk through the portal at a designated pace such as about ¼ the normalwalking speed. A millimeter wave transparent barrier 60 can be placed inthe portal as shown in FIGS. 5 and 6 in order to assure the properpositioning of the persons relative to the antenna elements. Varioustradeoffs are possible in the selection of the focal position of theantenna elements. In the system described an 18-inch focus wasspecified. Positioning the antenna optically farther away from thepersons being screened and increasing the focal length can providegreater depth of focus of the antenna elements. This can be done withmirrors 61 as shown in FIG. 6 to keep the unit compact. In FIG. 7 anestimate is provided of the approximate transverse resolution of thescanner as a function of distance of the between the surface beingimaged and the antenna elements.

While the present invention has been described above in terms ofparticular embodiments, persons skilled in the art will recognize thatmany other changes may be made. For example, infrared or visible camerassynchronized with the millimeter wave screener may be adapted to providecorrelated identity and reference information. Better resolution couldbe achieved by providing automatic focusing of the antenna elements.Alternatively, additional sets of elements could be provided withvarious focal lengths with processor software programmed to select thebest focus for each portion of the target person as he/she passesthrough the portal. Increasing the size of the antenna could alsoimprove the resolution. The person passing through the portal could berotated before a single stack or they could be rotated before the fourstacks. For applications where plenty of time is available a singleelement or fewer elements could be scanned across a person beingscreened, either automatically or by hand. Modifications to theamplifier shown in FIG. 4 could be made but preferably gains of at least50 dB should be provided. Therefore, the scope of the present inventionshould be determined by the appended claims and their legal equivalents.

1. A millimeter wave imaging system comprising: A) at least one millimeter wave frequency scanning antenna for collecting frequency dependent beams of millimeter wave radiation from a narrow one-dimensional field of view; B) a millimeter wave amplifier for amplifying at the collected frequencies said millimeter wave radiation; C) a beamformer for separating said amplified collected radiation to produce frequency dependent signals corresponding to said frequency dependent beams, said beamformer comprising: 1) a plurality of delay lines, 2) a millimeter wave lens, and 3) a plurality of millimeter wave power detectors; and D) a sampling circuit for reading out frequency dependent signals to produce a one-dimensional image of the antenna field of view.
 2. The imaging system as in claim 1 and also comprising a focusing means for focusing said frequency-scanning antenna.
 3. The imaging system as in claim 2 wherein said focusing means comprises a cylindrical reflector and a cylindrical lens.
 4. The imaging system as in claim 1 wherein said millimeter wave amplifier comprises three MMIC amplifiers fabricated on an indium phosphate substrate and a band pass filter.
 5. The imaging system as in claim 4 wherein said amplifier comprises a co-planar waveguide design.
 6. The imaging system as in claim 4 wherein said amplifier provides gains of at least 50 dB.
 7. The imaging system as in claim 1 wherein said delay lines are comprised of etched copper to create circuit patterns of varying lengths.
 8. The imaging system as in claim 7 wherein said delay lines define copper surfaces having surface roughness less than 300 nanometers.
 9. A portal contraband screener comprising a plurality of millimeter wave sensors, each of said plurality of millimeter wave sensors comprising: A) at least one millimeter wave frequency scanning antenna for collecting frequency dependent beams of millimeter wave radiation from a narrow one-dimensional field of view; B) a millimeter wave amplifier for amplifying at the collected frequencies said millimeter wave radiation; C) a beamformer for separating said amplified collected radiation to produce frequency dependent signals corresponding to said frequency dependent beams, said beamformer comprising: 1) a plurality of delay lines, 2) a millimeter wave lens, and 3) a plurality of millimeter wave power detectors; and D) a sampling circuit for reading out frequency dependent signals to produce a one-dimensional image of the antenna field of view.
 10. The screener as in claim 9 wherein each of said plurality of millimeter wave sensors also comprises a focusing means for focusing said frequency-scanning antenna.
 11. The screener as in claim 10 wherein said focusing means comprises a cylindrical reflector and a cylindrical lens.
 12. The screener as in claim 11 wherein each of said millimeter wave amplifiers comprises three MMIC amplifiers fabricated on an indium phosphate substrate and a band pass filter.
 13. The screener as in claim 12 wherein said amplifier comprises a co-planar waveguide design.
 14. The screener as in claim 12 wherein said amplifier provides gains of at least 50 dB.
 15. The screener as in claim 9 wherein each of said delay lines are comprised of etched copper to create circuit patterns of varying lengths.
 16. The screener as in claim 15 wherein said delay lines define copper surfaces having surface roughness less than 300 nanometers.
 17. A portal contraband screener comprising a plurality of millimeter wave sensors, each of said plurality of millimeter wave sensors comprising: A) at least one millimeter wave frequency scanning antenna for collecting frequency dependent beams of millimeter wave radiation from a narrow one-dimensional field of view; B) a fast switch for calibration; C) a millimeter wave amplifier for amplifying at the collected frequencies said millimeter wave radiation, said amplifier comprising at least three MMIC amplifiers fabricated on an indium phosphate substrate and a band pass filter; D) a beamformer for separating said amplified collected radiation to produce frequency dependent signals corresponding to said frequency dependent beams, said beamformer comprising: 1) a plurality of delay lines, 2) a millimeter wave lens, and 3) a plurality of millimeter wave power detectors; and E) a sampling circuit for reading out frequency dependent signals to produce a one-dimensional image of the antenna field of view, F) focusing means for focusing the sensor.
 18. The screener as in claim 17 wherein said focusing means comprises a cylindrical reflector and a cylindrical lens.
 19. The screener as in claim 17 wherein said amplifier comprises a co-planar waveguide design.
 20. The screener as in claim 17 wherein said amplifier provides gains of at least 50 dB.
 21. The screener as in claim 17 wherein each of said delay lines are comprised of etched copper to create circuit patterns of varying lengths.
 22. The screener as in claim 21 wherein said delay lines define copper surfaces having surface roughness less than 300 nanometers.
 23. The screener as in claim 17 wherein said plurality of sensors is arranged in four stacks each stack comprising at least 16 sensors. 